There is a continuing desire to commercialize superconducting rotating machines because superconducting motor and generator operation can result in relatively high efficiencies above 99% and more than an order of magnitude very high specific power than conventional machines. Use of High Temperature Superconductor (HTS) materials, i.e., materials which exhibit superconduction at temperatures above 30 K, for superconducting machinery has been demonstrated, and such conduction is referred to herein as conduction in a HTS state. To improve the potential for commercial use of such machines numerous efficiency limitations should be overcome. Note, for example, losses occur during delivery of power from a 300 K environment to the superconducting windings. In motor drive applications it is also necessary to generate and replenish current in the HTS windings. HTS conductors differ from low temperature superconductors in that it is not possible to establish a true persistent current loop with an HTS conductor. This is due to several factors including finite losses characteristic of HTS materials and characteristic resistive losses across splice junctions present in the circuitry. These small but significant losses must be considered when creating or restoring current levels with flux pump circuitry.
In the past flux pumps have been of two varieties. It has been possible to directly feed a charging current into a cryogenically cooled loop from a power supply operating in a normal mode. Losses through the conductor extending across the interface are significant. Alternately, flux pump transformers can transfer power into cryogenically cooled environment while maintaining substantial thermal isolation. However, flux pump transformer designs must provide rectification of the ac transformer signal to charge the low loss direct current loop needed in the rotor winding. It is desirable to provide a system incorporating such a flux pump function which is effective during various modes of machine operation.